Biological Conservation 58 (1991) 1-18

Disturbance, Equilibrium, and Environmental Variability: What is 'Natural' Vegetation in a Changing Environment?

D o u g l a s G, Sprugel

College of Forest Resources, AR-10, University of Washington, Seattle, Washington 98195, USA

(Received 16 May 1990; revised version received 29 November 1990; accepted 6 December 1990)

A B S T R A C T

To most early ecologists, the 'natural' ecosystem was the community that would be reached after a long period without large-scale disturbance (fire, windstorm, etc.). More recently, it has been realized that in most areas some type of large-scale disturbance is indigenous, and must be included in any realistic definition of 'naturalness'. In some areas an equilibrium may exist in which patchy disturbance is balanced by regrowth, but in others equilibrium may be impossible because (1) individual disturbances are too large or infrequent; (2) ephemeral events have long-lasting disruptive effects; and~or (3) climate changes interrupt any movement toward equilibrium that does occur. Examples of non-equilibrium ecosystems include the African savannas, the Big Woods of Minnesota, the lodgepole pine forests of Yellowstone National Park, and possibly the old-growth Douglas-fir forests of the Pacific Northwest.

Where an equilibrium does not exist, defining the 'natural' vegetation becomes much more challenging, because the vegetation in any given area would not be stable over long periods of time even without man's influence. In many areas it may be unrealistic to try to define the natural vegetation for a site; one must recognize that there are often several communities that could be the 'natural' vegetation for any given site at any given time.

I N T R O D U C T I O N

Much attention has recently been paid to attempts to maintain 'natural ' conditions in wilderness or natural areas, especially in the presence of

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Biol. Conserv. 0006-32707/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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frequent disturbance such as fire and windstorms. This has led, not surprisingly, to controversy (Bonnicksen & Stone, 1985; Parsons et al., 1986; Pyne, 1989) and even acrimony (Bonnicksen, 1989) over exactly what is meant by the 'natural' conditions for any given area--and indeed, over the very definition of 'natural'. Is the 'natural' vegetation what the first white explorers saw? the first settlers? writers? photographers? or the first plant ecologists? And does the specific date when the vegetation was first described make a difference? Would the first European explorers have seen the same thing if they had reached the eastern US in the 1300s instead of the 1500s? What benchmark--if any--can we use to define the 'natural' condition of the landscape?

A BRIEF HISTORY OF NORTH AMERICAN ECOLOGICAL IDEAS ABOUT NATURAL DISTURBANCE

Much of the controversy about 'natural' conditions relates to the role of natural disturbance in natural ecosystems, which has been the subject of much interest and wide swings of opinion throughout the history of American plant ecology. One of the first American ecologists to discuss disturbance as a normal component of some natural ecosystems was W. S. Cooper, who described the forest vegetation of Isle Royale as 'a complex of windfall areas of differing ages... [that] changes continually in a manner that may almost be called kaleidoscopic when long periods of time are considered' (Cooper, 1913).

Although Cooper considered recurrent natural disturbance as a normal and inevitable component of community structure and function, his appreciation of disturbance as a natural ecosystem component was not shared by many of his early colleagues. Most ecologists in the first half of the 20th century believed that ecosystems typically progressed steadily and predictably along well-defined successional pathways until they reached a stable, self-sustaining state ('climax'), which was the 'normal' condition for communities in that geographic region. While it was impossible to ignore the fact that the vastlmajority of the inhabited earth was covered by successional ecosystems, most ecologists believed that this was due solely to man's pervasive burning, clearing, and plowing. F. E. Clements and J. E. Weaver, who dominated ecological thinking in America in the 1920s and 1930s (Tobey, 1981), wrote in their classic textbook: 'As a consequence of almost universal use and misuse by man, subseres [successional communities developing after disturbance] in every possible stage of succession constitute the most abundant of all communities.. . In regions long settled, subseres form practically the entire cover of vegetation' (Weaver & Clements, 1938).

Environmental variability and 'natural' vegetation 3

But: 'Before the advent of civilized man, nearly the whole area of each climax was occupied by the [climax] dominant species.' (Weaver & Clements, 1938).

There were objections to this well-ordered view of the world even in the heyday of Clementsian ecology (Sernander, 1936; Graham, 1941), and these objections became louder and more persistent in the 1950s and 1960s (Watt, 1947; Drury, 1956; Lutz, 1956; Biswell, 1961; Rowe, 1961). But the orderly succession-to-climax paradigm remained dominant in most ecological thinking and writing (and especially in writing for the lay public) well into the final third of the 20th century.

The pendulum of ecological opinion finally swung toward a recognition of the importance of natural disturbance in the early 1970s. At that time, a flood of papers more-or-less simultaneously proclaimed the importance of natural disturbance in chaparral (Hanes, 1971; Biswell, 1974), northern boreal forests (Heinselman, 1971, 1973; Rowe & Scotter, 1973), western conifer forests (Habeck & Mutch, 1973; Kilgore, 1973; Loope & Gruell, 1973), eastern conifer forests (Sprugel, 1976; Reiners & Lang, 1979; Fig. 1), deciduous forests (Loucks, 1970; Bormann & Likens, 1979), tropical forests (Whitmore, 1974, 1975), rocky intertidal zones (Dayton, 1971; Levin & Paine, 1974) and a variety of other ecosystems (White, 1979). In each of these ecosystems, the authors said, natural disturbance is so common that it keeps the system from ever reaching a stable state, so it is unrealistic to assume that climax is the 'normal' condition for ecosystems to be in. Most of the systems, it turned out, were akin to Cooper's Isle Royale forests--a 'continually

Fig. 1. Wave-regenerated forest on Whiteface Mt, New York.

4 D.G. Sprugel

changing mosaic or patchwork' of patches of different ages, with disturbance normally intervening before any patch reaches a stable condition.

However, although the pendulum swung away from the orderly 'climax' paradigm toward one featuring recurrent but unpredictable disturbance, the notion of ecosystem stability was retained at a higher level. Several authors (e.g. Heinselman, 1973; Wright, 1974; Sprugel, 1976; Van Wagner, 1978; Bormann & Likens, 1979; Shugart & West, 1981) emphasized that even ecosystems with a high disturbance frequency could be in a 'steady-state' or 'equilibrium' if the creation of new patches was balanced by the maturation of old ones. A small patch might give the impression of constant change, but on a larger spatial scale (soon to be called a landscape) a perceptive observer would observe a balance between disturbance and succession. Thus the basic notion of a stable ecosystem still prevailed, but stability was expected to occur at a larger spatial and temporal scale. Cooper would doubtless have been pleased, since the new ideas about 'equilibrium landscapes' were really just restatements of his contention that 'the balsam-birch-white spruce forest, in spite of appearances to the contrary, is, taken as a whole, in equilibrium.., the changes in various parts balancing each other' (Cooper, 1913).

EQUILIBRIUM AND NON-EQUILIBRIUM LANDSCAPES

The idea of an area maintained in a dynamic equilibrium by a balance between disturbance and recovery is psychologically attractive, because it provides some sense of stability even in the presence of constant change. Although it is difficult to define an equilibrium landscape precisely, most discussions regard it as one in which some parameter of interest (species composition, biomass, net primary productivity, etc.) is roughly constant from year to year when averaged over the whole landscape, or in which opposing processes (e.g. gross primary production vs total respiration, or nutrient input vs nutrient loss) are approximately balanced on a landscape scale (Bormann & Likens, 1979; Shugart, 1984). In any real ecosystem there will be some year-to-year variation in all these parameters, but most authors have considered these small-scale variations unimportant. What is more critical is whether there are long-term trends, which in turn depends on whether or not the proportion of the landscape in major developmental stages varies substantially over time periods on the order of a decade or a century. If that occurs, then populations of species that depend on different developmental stages, or processes that are accelerated or diminished there, will not be even approximately constant on a landscape scale.

This general definition of an equilibrium landscape may well be met

Environmental variability and 'natural' vegetation 5

where natural disturbances are frequent and small compared with natural landscape units. One well-documented example of this situation is the deciduous forest of eastern North America, where the predominant type of natural disturbance seems to be windthrow of individual trees or small groups (Bormann & Likens, 1979). Since a windthrow typically affects a few hundred to l oo0m 2 (Runkle, 1982), equilibrium could theoretically be reached in an area of less than 1 km 2. Bormann and Likens (1979) have described the resultant patchwork of gaps of various ages as the 'Shifting- Mosaic Steady State'. Another example is the wave-regenerated balsam fir Abies balsamea forests of the northeastern US, where natural 'mortality waves' move through the forest once every 50-70 years (Sprugel, 1976; Sprugel & Bormann, 1981). Since the waves move at a fairly uniform rate, there are always freshly killed areas, and since consecutive waves are only about 100m or so apart, an area of a few dozen hectares will normally contain all phases of the disturbance cycle (Figs 1 and 2). By almost any criterion either of these ecosystem types is in an equilibrium or quasi- equilibrium state; disturbance is sufficiently frequent and small-scale compared to the landscape area that most populations and processes must be fairly constant over the whole area, and there is little reason to expect long-term trends under the current climatic regime.

Non-equilibrium due to spatial scale of disturbances

Although the notion of equilibrium landscapes is attractive and satisfies human longing for order in natural processes, it is becoming increasingly clear that not every landscape is in or can reach an equilibrium state (Delcourt et aL, 1982). The most obvious example of non-equilibrium is where the typical spatial scale of disturbance approaches or exceeds that of

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MATURE DYING- I YOUNG- REGENERMING t INTERMEDIATE MMURE

Fig. 2. Diagrammatic view of a fir wave.

6 D.G. Sprugel

landscape units as generally understood (Franklin & Hemstrom, 1981; Shugart & West, 1981; Shugart, 1984). Wherever individual disturbances are so large that a single disturbance event can affect a relatively large proportion of the landscape, achievement of an equilibrium state is unlikely; there will inevitably be wide swings from one decade or century to the next in the proportion of the landscape in different developmental stages. An example might be the low-elevation forests of the Pacific Northwest, where in presettlement times individual fires may have covered hundreds of thousands of hectares (Franklin & Hemstrom, 1981; Pyne, 1982). Eastern forests along major hurricane tracks (Smith, 1946) are another example; in both cases, even if one considers a landscape the size of an entire state, it is difficult to see how any kind of equilibrium could become established where disturbances are so large. Even before the advent of settlement and logging, there must have been large changes from decade to decade or century to century in species populations and rates of ecological processes as the relative proportion of different-aged patches varied over time.

Non-equilibrium due to unique events in the past

One-time events, both natural and anthropogenic, may also have long- lasting impacts on vegetation. A well-documented example is the appearance of a novel pathogen of eastern hemlock Tsuga canadensis about 4800 years ago, which caused a sharp decline in hemlock populations throughout the species' range (Davis, 1981). It took almost two millenia before hemlock populations returned to their pre-decline levels. A more recent example whose importance has yet to be fully evaluated is the devastation wrought by European diseases (especially smallpox) on American Indian populations in the 16th and 17th centuries (Crosby, 1986). Since Indians interacted strongly with local plant communities (e.g. by setting fires; Grimm, 1984), a decline in Indian populations must have had a sharp impact on local vegetation. It is quite likely that in many areas the vegetation was still re-equilibrating to changed Indian impacts when the first white settlers arrived.

Non-equilibrium due to climatic variability

Finally, a landscape may remain out of equilibrium because of climate changes. Humans, as relatively short-lived organisms (compared to trees), usually think of climate and other environmental characteristics as fairly constant. We recognize succession as a dynamic play, but regard the stage on which it is played out as static and unchanging. When we do think about past climate change, we usually think in terms of the 'big' climate shifts such as

Environmental variability and "natural" vegetation 7

the Pleistocene glaciation, and assume that changes that occur on a shorter time scale have little effect on vegetation structure and function. But this presumption of environmental stability is not necessarily accurate. Substantial natural climate changes, large enough to cause significant changes in vegetation distribution and prevent the achievement of equilibrium, occur on a time scale that is well within the lifespans of forest trees (Delcourt et al., 1982; Davis, 1984). For example, 800-1000 years ago the climate was so warm that Vikings were able to colonize Greenland, and vineyards were planted in England (Lamb, 1982). While a thousand years may seem a long time in human terms, it is less than one-third of the lifetime of a giant sequoia.

The examples that follow will illustrate the kind of changes that can and have occurred in ecosystems which appear to be stable, and which have a direct effect on the kind of vegetation we see today. (While these examples, like those above, are drawn primarily from North American temperate forests, the principles they illustrate apply to a wide variety of ecosystems around the world.) These examples will also illustrate just how incorrect our perceptions of 'stable' vegetation can be.

Example 1: The African savannas One of the most dramatic images in popular biology is the African savanna, with its expansive vistas, impressive large grazers, and peculiar umbrella- shaped trees. To many people this represents unspoiled nature at its finest. However, there is good evidence that the trees, at least, are not 'natural'; despite their conspicuousness now, they were not there (at least not so widespread) 100 years ago and (if present trends continue) will not be there 100 years from now. The widespread presence of trees (mostly Acacia tortilis) over large areas of the savannas seems to be a direct result of the introduction into Africa of the cattle disease rinderpest, in about 1895 (Sinclair, 1979). Rinderpest caused catastrophic mortality among native and introduced ungulates; its effects on the ecosystem were complex and will not be detailed here, but the ultimate effect was that trees that had previously been restricted to protected areas spread rapidly into areas where tree establishment had been prevented by a combination of grazing and burning (Fig. 3). Later, as resistance to rinderpest developed in native ungulates and vaccines were developed to protect domestic livestock, tree establishment was again suppressed, so few new trees have become established in the last fifty years. As a result, there are now few sapling-sized trees to take the place of the 60-80-year-old cohort that is now dying. This has created extraordinary problems for the park and reserve managers of the region; visitors to East Africa (who provide the money that justifies the parks' existence) expect to see big, umbrella-shaped trees, and are disappointed if

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RINDERPEST (introd. 1985)

t 95 % mortality of wild and domestic ungulate~

S / ~ reduction in reduction in grazing hungry lions -~ man-eating llons----.,~_ and brow~g 7 a n populations

fewer ignitions; lower fire frequency

in tree recruitment

I.ove.opmen, o..eo e woo.,.n, learl 'O0'sl]

Fig. 3.

(ca. 1920) Resistance develops Woodland among wild u n g u l a t e ~ y a n o p l e s close

Tree seedling recruitment declines

Idevelopment of open woodland I

Effect of the introduction of rinderpest on tree populations in East Africa. Based on Sinclair (1979) and Norton-Griffiths (1979).

they do not. Regardless of the fact that the plains without trees may be more 'natural' than plains with trees, our image of the savannas has become fixed in a static early 20th century mold.

The fundamental problem for park managers, then, is not that the trees of the African savannas are dying, but that visitor expectations have become fixed around an apparently stable vegetation type that was actually ephemeral. To quote Pellew (1983): 'The woodland structure desired by park management is an unstable transition stage.. , which has been adopted by management as the status quo because it happened to be present in the good old days of ten years ago'.

Example 2: The Big Woods of Mhmesota When early French trappers arrived in southeastern Minnesota, they were so impressed by the grandeur of the forests that they called them 'Bois Fort ' or 'Bois Grand'. Early American plant ecologists were equally impressed; the noted ecologist R. P. Daubenmire wrote his doctoral dissertation and a now-classic paper on the few fragments of Big Woods that remained in the 1930s, and determined that the maple-elm-basswood forests that occurred there represent the climax formation for the region--that is, the vegetation

Environmental variability and 'natural" vegetation 9

toward which all areas in the region would tend given enough time (Daubenmire, 1936).

One would assume that a mesic, climax forest covering several thousand square kilometers and impressive enough to be called the 'Big Woods' must be an old and well-established vegetation feature. Such an assumption would be wrong. Daubenmire noted that much of the Big Woods was underlain by prairie soils, and pollen analysis (Grimm, 1983, 1984) has demonstrated that the Big Woods actually developed only about 300 years ago--hardly long enough for any kind of equilibrium involving long-lived trees to be established. Before about 1650 the area was covered by oak woodlands, in which frequent fires apparently restricted elm Ulmus americana, sugar maple Acer saccharum, basswood Tilia americana and other fire-intolerant mesic species to unusually moist and protected sites. It appears that during the 17th century a climatic change--possibly associated with the onset of the coldest part of the 'Little Ice Age'--tipped the balance toward mesic forests by reducing fire frequencies enough to permit mesic species to invade (Grimm, 1983; Clark, .1988). Once established, elm-maple- basswood forests do not burn easily (Grimm, 1984), so they were apparently able to maintain themselves even after climates warmed up again. (We will never know how much longer the Big Woods species would have been able to retain their hold on these sites, because they have now been almost completely converted to farms and housing developments.) Thus, like the umbrella-trees of the Serengeti, the solid, stately 'Big Woods' were a much more recent vegetative feature than one might think at first glance, and may have owed their existence to a relatively transient environmental event.

Example 3: The lodgepole pine forests of Yellowstone Park The lodgepole pine Pinus contorta var. latifolia forests of Yellowstone Park are superb examples of the natural forests of the northern Rocky Mountains and have been relatively little altered by the activities of man. Since the forests were rarely used by Indians and have never been logged, one might expect that if an equilibrium landscape could develop anywhere in the western US, Yellowstone would be a likely place to look for it. (Note that, as was suggested earlier, an equilibrium landscape is not one in which the individual stands are stable--in a lodgepole pine forest this would be unlikely--but rather one in which the changes in different parts of the landscape balance each other so that the overall effect is one of no net change.) However, Yellowstone Park is not in anything like an equilibrium, and probably never has been. Romme, and Despain (1989) mapped the disturbance history of a large part of the park and found that in the 1980s it was heavily dominated by 250- to 300-year-old stands, with forests less than 250 years old much less common. In 1988 the park was struck by massive

10 D.G. Sprugel

fires, so now instead of old forests, young and regenerating forests cover wide areas of the Park- -bu t the lack of an equilibrium persists.

The proximal explanation for this chronically unbalanced age distri- bution seems to be that much of what was to become the Park was swept by fires in the 18th century, and that regenerating lodgepole pine forests do not burn readily until they are at least 200-250 years old (Romme, 1982). (For this reason, Romme and others doubt that human fire exclusion policies have actually had much effect on fire frequency in Yellowstone, since so much of the area was in a relatively non-flammable state until the last few decades.) It then remains to ask why such a large area burned in the mid-18th century. Possibly it is another case where spatial scale prevents equilibrium; the natural size of fires in high-altitude lodgepole pine forests may be so large that the entire burned area represents a handful of large fires that coincidentally occurred within a few years of each other. However, another possibility is that fire intensity is closely tied to weather conditions, so that in exceptionally dry years (such as 1988), fires start throughout the park and the surrounding region and burn over wide areas. If this is true, then a few drought years in the early 1700s may have synchronized the forests so that they all reached a flammable state at about the same time. The massive fires of 1988 may not have been due to climate alone, but rather to an interplay of climate, topography and forest development patterns that locks the high- altitude forest into a long cycle of synchronized development alternating with wide-ranging fires, so that the system never approaches equilibrium.

Example 4: The old-growth forests of the Pacific Northwest The old-growth forests of the Pacific Northwest are some of the most awe- inspiring forests in the world. The immense size of the trees and the cool quiet mustiness of the forest floor evoke a sense of the Forest Primeval that few ecosystems anywhere can match. And it is certainly true that many stands are very old; stands dating back to the 13th and 14th centuries are common in the remaining old-growth areas. But since many northwestern trees can live over 1000 years, the fact that the forests are old is no guarantee that they have reached an equilibrium. So it is still reasonable to ask: were the old-growth forests of the Pacific Northwest in an equilibrium state before logging began to take its toll?

The answers to these questions are less clear than in the previous examples, because the forests are so large and difficult to study, and because they have been so fragmented by logging and other development. However, it is well known that most of the 'old-growth' forests are dominated by fire- following successional species (mainly Douglas-fir Pseudotsuga menziesii) rather than the more shade-tolerant species that would dominate after a prolonged period without fire or other large-scale disturbance. But the

Environmental variability and 'natural" vegetation 11

limited evidence available suggests that in recent centuries the fire frequency has been very low; Hemstrom and Franklin (1982) calculated a natural fire rotation of about 450 years for the presettlement forests of Mt Rainier National Park. With such a long fire interval, one would expect the forests to include a substantial proportion of very old stands, but this does not seem to be the case. In Mt Rainier National Park, for example, less than 2% of the forests are older than go 1230 (Hemstrom & Franklin, 1982). If the presettlement forests had been in equilibrium, then according to the negative exponential model that is normally used to predict equilibrium stand-age distributions in disturbance-prone ecosystems (Van Wagner, 1978) a 450-year natural fire rotation would mean that about 17% of the forests should have been more than 800 years old. In another example, Yamaguchi (1986) aged 55 very old trees spread over a 600km 2 area northwest of Mt St Helens and found that 50 of them had established between 1300 and 1350, but only five before 1300--again, a somewhat surprising lack of the very old trees and stands one would have expected scattered through an equilibrium forest. This does not simply reflect difficulty in dating very old forests due to death of the original colonizers, since Douglas-fir is quite capable of living 1000-1200 years (Franklin & Waring, 1980; Hemstrom & Franklin, 1982). Rather, it seems to reflect a real anomaly; while 600-700-year-old stands were comparatively common before logging, there were simply not as many pre-13th century forests as one might expect in an equilibrium situation.

As with the establishment of the Big Woods, there is a reasonable climatic explanation for this pattern. As was noted earlier, during the 'Medieval Optimum' (AD 1000-1300), climates in many parts of the Northern Hemisphere were substantially warmer than they are now. T. W. Swetnam (1989, pers. comm.; Swetnam et al., 1990) has found that the fire frequency in Sequoia-Kings Canyon National Park (California) was much higher in this period than it was in the following centuries. While there is no proof that the Pacific Northwest was also unusually warm and dry during this period, it may have been, and if so one would expect that fire frequency in North- western forests would also have increased. For example, Hemstrom and Franklin (1982) deduced that about 47% of the forested area in what is now Mt Rainier National Park burned in 1230. Thus there may be few pre-13th century stands simply because almost no stands survived the Medieval Opt imum unburned. The cooling trend in the early 1300s that ended the Viking venture in Greenland may have brought a drop in fire frequency which permitted the establishment of the giant forest we see today.

The apparent disruption of the old-growth stands 800 years ago was not a unique event interrupting a long period of stability. Indeed, the pollen record suggests that old-growth Douglas-fir forests as we know them probably did

12 D. G. Sprugel

not exist until about 6000 years ago; before that, stands with the structure and species composition of today's old-growth stands either did not exist or occurred only in small protected areas (Brubaker, 1991). Thus the majestic old-growth vegetation type itself has been in existence for only about 5-10 Douglas-fir life-spans.

It seems likely, then, that even before logging began the old-growth forests were not in equilibrium with their disturbance regime, and probably never would have been. In the late 1800s they were still showing the effects of a warmer, drier climate almost a thousand years ago, and before the effects of that perturbation could be damped out another climate change would probably have altered fire frequencies again.

DISCUSSION

These examples of vegetational instability suggest at least four basic points about vegetational equilibria:

(1) Many types o f natural vegetation are far less stable than they appear to be Because trees live so much longer than humans, there is a natural tendency to think of any tree-dominated ecosystem as long-lasting and stable. The four examples cited above illustrate that even solid, firmly established vegetation types are often less permanent than one might think. The umbrella trees that are so distinctive in the African savannas have been there less than 100 years, and are the result of human introduction of a devastating exotic pathogen. The Big Woods are only a little older, and may result from a relatively minor climatic change in the 17th century. The Yellowstone forests undergo wide swings in average maturity; John Colter, the first white man to visit Yellowstone, saw a much different ecosystem in 1807 than the tourists in the mid-20th century. And although the old-growth forests of the Pacific Northwest create the impression of great age and stability, they seem to be the products of relatively recent disturbance; many are apparently still recovering from a period of higher fire frequency around the time of the Norman Conquest, and the vegetation type itself has only been around for a few millenia. In all of these cases, then, the current ecosystems are more variable or more ephemeral than they might seem at first glance.

(2) Small or transient environmental changes can cause large and long-lasting vegetation changes In the long run, vegetation distribution is remarkably sensitive to climate, and a comparatively small change in mean temperature or precipitation can cause a large change in vegetation. For example, the coldest part of the Little

Environmental variability and 'natural' vegetation 13

Ice Age, which apparently permitted the development of Big Woods vegetation in southern Minnesota, was only about 1-1-5°C cooler than the preceding period (Lamb, 1982). Indeed, even during the peak of the Pleistocene glaciations, mean air temperatures in the Northern Hemisphere were only about 5-6°C lower than those at the beginning of the 20th century (COHMAP, 1988). Long-lived vegetation may be slow to respond to a climate change, but given enough time even small climate variations can lead to large vegetation changes.

One of the most significant results of small (1-2°C) climatic changes may be their effect on disturbance regimes (Grimm, 1984; Clark, 1988; Swetnam et al., 1990). In particular, cool conditions reduce fire frequency, and warm conditions increase it. Thus Clark (1988) found that fire in north- ern Minnesota recurred about once every 44 years before the Little Ice Age began, but only once every 88 years after it had started. These changes in turn can affect vegetation structure and distribution, and indeed may be the main way in which small climate changes alter vegetation. This was probably the case in the transition from oak woodland to Big Woods in southern Minnesota; lower temperatures and higher rainfall would probably not have been sufficient by themselves to give maple and basswood a competitive advantage over oaks, but they apparently reduced fire frequency enough so that fire-intolerant mesic species were able to gain a foothold in areas where only fire-tolerant species were formerly able to survive (cf. Grimm, 1984).

(3) Every point in time is special The 1963 report of the Advisory Committee on Wildlife Management in the National Parks, usually known as the Leopold Report, recommended that in Park management 'above all other policies, the maintenance of naturalness should prevail', and more specifically, that 'the biotic association within each park should be maintained, or where necessary recreated, as nearly as possible in the condition that prevailed when the area was first visited by the white man'. As we learn more about vegetation history it becomes increasingly obvious that, while the general idea of trying to preserve vegetation in a 'natural' state might be desirable, identifying a specific point in time as epitomizing the 'natural' state is ill-advised, particularly for non-equilibrium systems (Johnson & Agee, 1988; Christensen, 1988). The Yellowstone stiuation provides an excellent example: since the majority of the Park burned over in the 18th century, perpetuating the condition--including the wildlife and fish populations-- that prevailed when the area was seen by the first white man (1807) would require a massive program of prescribed burning to keep the forest dominated by young, regenerating stands. This is surely not what the Leopold

14 D.G. Sprugel

Committee had in mind, and it points out a generic problem: at any specific point in time, the vegetation of any area has some special characteristics that make it different from other times that might equally well have been chosen.

(4) Because of vegetational instability, it may be impossible to define the natural vegetation or the natural disturbance regime in many areas When it comes to defining 'natural' vegetation, the present is little help; nearly all ecosystems have been altered by fire or predator control and exotic species introductions. And the past provides at best an equivocal guide to what the present might have been, since climatic variability, change events, and the simple passage of time might have brought dramatic changes even in the absence of man. For example, the Big Woods might have reverted to oak savanna during the drought of the 1930s (if it had not been converted to farmland and housing developments first), and even without man's efforts Yellowstone would have changed dramatically in the 19th and 20th centuries. Thus even if we knew exactly what the vegetation and disturbance regime were like two or five hundred years ago, there is no guarantee--or even any particular reason to expect--that without man's influence they would have been the same today.

The problem is not just one of an inability to predict what might have happened in man's absence; rather, it is that in a non-equilibrium environment the whole notion of 'the' (unique) natural vegetation or disturbance regime is flawed. Because chance factors and small climatic variation can apparently cause very substantial changes in vegetation, the biota and associated ecosystem processes for any given landscape will vary substantially over any significant time per iod--and no one variant is more 'natural' than the others. Park and reserve managers and other ecologists concerned with 'natural' area management must recognize this, and understand that the range of 'natural' vegetation and processes is probably much broader than is commonly imagined. If a given vegetation type or disturbance regime occurred in an area at some time during the past 1000 years, how can one say it is not a (if not necessarily the) 'natural' type today (Christensen, 1988)?

Worster (1977) claimed that 'where the climax is ignored or distorted as an ideal, the only criterion left is the marketplace'. This view is not only short- sighted, ignoring as it does the role of disturbance as an important natural process, but also simply wrong. Clearly, not all possible biotic assemblages are 'natural'; a planted, fertilized, pesticide-saturated cornfield, or a forest freckled with clean, square clearcuts, is not a natural ecosystem by any reasonable definition. Paleoecological research can often determine the range of vegetation types and disturbance regimes that have occurred within

Environmental variability and 'natural' vegetation 15

the recoverable past, and such research can guide management activities aimed at keeping the vegetation within the 'natural' range. The notion of 'natural' vegetation or ecosystem processes need not be abandoned as a goal for park or reserve management, even though it must be revised to recognize that there is a range of ecosystems that can legitimately be considered 'natural'.

ACKNOWLEDGEM ENTS

Dr L. B. Brubaker and N. L. Christensen stimulated me to think about the role of climatic variation in long-term vegetation dynamics, and the management implications of non-equilibrium natural ecosystems. Drs Brubaker and J. K. Agee read drafts of this manuscript and provided useful comments. Salary support during the development of the ideas and the preparation of the manuscript was provided by grants from the National Science Foundation, the US Department of Agriculture Competititive Grants program, and the US Environmental Protection Agency.

REFERENCES

Biswell, H. H. (1961). The big trees and fire. National Parks, 35, 11-14. Biswell, H. H. (1974). Effects of fire on chaparral. In Fire and Ecosystems, ed. T. T.

Kozlowski & C. E. Ahlgren. Academic Press, New York, pp. 321-64. Bonnicksen, T. M. (1989). Nature vs man(agement). J. For., 87(12), 41-3. Bonnicksen, T. M. & Stone, E. C. (1985). Restoring naturalness to national parks.

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